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Power supply circuit question - Transformerless good idea? Still pass certification?

Discussion in 'Power Electronics' started by Mahonroy, Sep 22, 2015.

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  1. Mahonroy

    Mahonroy

    57
    1
    Oct 21, 2014
    Hey guys,
    I am in the process of deciding on a good rout to take for the power supply for my project. I had a couple of questions about it.

    More info on the device: The device is for home brewing, its a standalone device that sits on the floor, shelf, mounted to a frame, etc. Its actually pretty simple - there is a power on/off, a screen, 3 buttons to interface the microcontroller/screen, a power cable, and that's pretty much it. The microcontroller uses transistors/relays to control a load (internal to the device) that requires 12 volts @ 20 amps.

    I have always just used power adapters/wall worts in the past for all of my projects, so this is a bit new to me. The highest current rated 12 volt power adapter I could find was @ 12 amps and it costs about 30 bucks (ebay). Here is a photo of it as an exampe:
    power_adaptor.JPG

    Something like this would of been perfect if it were offered as a 30 amp, but this doesn't seem to exist.
    So the next option is to use an actual 12 volt power supply, these are rated @ 30 amps and actually cost less (20 bucks). Here is a photo for an example:
    power_supply.JPG

    While this meets the specification, for a consumer product this just doesn't seem like the best idea, even if it were pre-wired (this would still need to be located outside of the device in a similar fashion as a power adapter would be, its a bit of an eye sore, and the electrical terminals are exposed - seemed like someone could get shocked?)

    So this brought me to my 3rd idea... what if I created a simple power supply that resided on the PCB inside the device? I found several plans online on how to make a transformer or transformerless power supply. The device has internal heat ventilation, so the added heat is not a problem. The problem I found with the transformer version was that the transformers are expensive - especially one that can handle enough current for 20+ amps. And to be honest, I actually had a hard time even locating an appropriate transformer for this. So I started looking into transformerless power supply circuits, and this seemed like the way to go.

    So it looks like these transformerless power supplies basically consist of a bridge rectifier circuit, paired up with some resistors, zener diods, and some even had a few IC's like the 78XX linear regulators incorporated, maybe some transistors, all of which could supply 12 volts @ 30 amps. It doesn't matter if the power supply is not very efficient, and that the power is not optimized and is dirty power. Its only going to be used to power a micro controller and the load, so dirty power doesn't matter (I would of course incorporate a few filtering circuits anyways).

    My main concern was mains isolation. With this device will it even matter? Would it be able to still get a certification? People aren't going to be exposed to any of the circuitry so it seemed like this would be ok. Mainly I wanted to get some of your opinions on this, or if there is a better idea that I should go with. Or if I should just go with the eye sore 30 amp power supply I mentioned above and have it sitting outside next to the device? Any help is greatly appreciated, thanks!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    those switchmode power supplies need to be properly encoded in a case.
     
    Martaine2005 and davenn like this.
  3. Old Steve

    Old Steve

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    Jul 23, 2015
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    As Steve said, it needs cased ... no big deal, what's the problem @Mahonroy ?

    they are awesome supplies, have used dozens of them over the yrs


    Ohhh and BTW they are NOT transformerless, the hot side is isolated from the secondary side by a transformer
     
  5. davenn

    davenn Moderator

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    Old Steve likes this.
  6. Old Steve

    Old Steve

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    Jul 23, 2015
    Yeah, you're right. I just threw it in because it came up in the search.
     
    davenn likes this.
  7. Mahonroy

    Mahonroy

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    Oct 21, 2014
    Thanks for the replies, I will do my best to answer all of the questions:

    1. Old Steve, I really liked that power supply schematic and something like that would work perfectly. The only problem is I cannot find a transformer with a 24V secondary rated for 35 amps. Well I did find a couple, but the transformers alone were over $120! Feels like I am missing something?

    2. Do the switchmode power supplies have to be in a case? Or is this for aesthetic reasons? If the naive customer was ok with it, would there be any problem? This is interesting though... it looks like a lot of manufacturers build enclosures so that you can put the power supply inside of it... seems a bit redundant I suppose.

    3. Davenn, I didn't mention that the switchmode power supply was transformer-less. I was asking about different design concerns if I was to go the rout of building a transformer-less power supply directly on my PCB that is already inside of the device.

    Thanks again!
     
  8. Old Steve

    Old Steve

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    Jul 23, 2015
    No, you're not missing anything - it's @#$% expensive to make your own high-power linear PSU.
    A large toroid is very hard to wind, so very expensive. As mentioned by the others, a switchmode PSU is really the way to go.
     
  9. davenn

    davenn Moderator

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    NO, DONT do transformerless
     
  10. Mahonroy

    Mahonroy

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    Oct 21, 2014
    I need to know why though.
     
  11. davenn

    davenn Moderator

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    cuz its freakin' dangerous because of the total lack of isolation
     
    Old Steve likes this.
  12. Alec_t

    Alec_t

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    Jul 7, 2015
    .... and because of the huge amount of heat generated (as already mentioned above)!! Your pcb would catch fire in no time :eek:.
     
    Last edited: Sep 22, 2015
  13. Martaine2005

    Martaine2005

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    May 12, 2015
    Heed *steve*s' advice in post#2.
    Buy a nice looking enclosure.
    Simples..

    Martin
     
  14. BobK

    BobK

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    Jan 5, 2010
    Transformerless power supplies are practical for a few milliwatts, not 360W! And they can only be used when completely enclosed with no part of the circuit exposed since the entire circuit is at line potential and therefore a shock hazard.

    Bob
     
    (*steve*), davenn and Arouse1973 like this.
  15. Minder

    Minder

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    Apr 24, 2015
    That must be one aimed specifically for home brewing! :D
    Worts - Warts :p
    M.
     
    Alec_t likes this.
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